This species is one of the rarest and most ancient frogs in the world. It was originally found on both the North and South Island of New Zealand, but its range was reduced to a 600 m sq. boulder bank near the summit of Stephens Island. It is a ground-dwelling and night-active species that hides in damp, mossy rock crevices during the day. Males guard the nesting site and carry the newly hatched young on their backs. Despite being less than 50 mm long, it is New Zealand’s largest native frog. It is threatened by its tiny range and small population, introduced and native predators, and potentially a virulent fungal disease (chytridiomycosis).

Urgent Conservation Actions

Coninuation of the wide variety of conservation actions are ongoing for this species; establishment of a captive breeding project.

Hamilton’s frog is the largest native frog species in New Zealand, reaching a total length of almost 50 mm. This species is still very small relative to the world’s largest frog, the Goliath frog of West Africa, which can reach body lengths of 400 mm and weigh 3.1 kg. Hamilton’s frog is also less than half the size of an extinct species of native frog (Leiopelma waitomoensis) which reached a total length of 100 mm, formerly the largest in New Zealand.

Hamilton’s frog is names in honour of Harold Hamilton, who first formally collected the species in the early twentieth century.

Populations of a New Zealand frog species on Maud Island in the Cook Strait were previously considered to be Hamilton’s frog, but were subsequently described as a new species, the Maud Island frog (Leiopelma pakeka – EDGE rank 58), in 1998 as a result of genetic analyses.

Research has shown that New Zealand frogs may find each other and/or communicate using visual clues or perhaps through pheromones. Studies have revealed this behaviour in two species of New Zealand frog – the Maud Island frog (EDGE rank 58) can respond to olfactory cues (messages detected through their sense of smell) in faeces, and Hochstetter’s frog (EDGE rank 38) prefers its own odour to the smell of unfamiliar members of the same species. This system probably developed in the absence of being able to communicate via sound, because New Zealand frogs lack both eardrums and developed vocal chords.

Evolutionary Distinctiveness

Order: Anura

Family: Leiopelmatidae

The prehistoric New Zealand frogs (of the family Leiopelmatidae) are the most ancient and primitive frogs in the world, diverging from all other frog and toad lineages over 200 million years ago in the former southern supercontinent of Gondwana. The ancestor of these species actually colonised New Zealand over 80 million years ago, when it was still part of the Gondwana, and the Leiopelmatidae shares a similar biogeographic distribution to several other ancient plant and animal groups, such as southern beeches, tuatara, and the giant extinct moa. This means New Zealand frogs started to evolve independently as a lineage before the formation of the Atlantic Ocean, over 50 million years before the first bird appeared in the fossil record. They are part of a small suborder of frogs called the “archaeobatrachia” or ancient frogs. The archeobatrachia comprise less than 7% of all of the frogs and toads, including some of the most evolutionarily distinct amphibian species. Each one therefore represents a disproportionately high amout of distinct evolutionay history in today’s biodiversity.

The four surviving species of New Zealand frog are found only in New Zealand, and these species are regarded as “living fossils” since they are very similar to frog fossils found in Queensland, Australia from the late Jurassic period around 150 million years ago. Fossil records actually show that New Zealand had seven leiopelmatid frog species just 1000–2000 years ago. The scientific names of the three extinct species are Leiopelma auroraensis, Leiopelma markhami and Leiopelma waitomoensis, which was previously the largest species of New Zealand frog at a total length of 100 mm – over twice the size of the largest New Zealand frog species alive today. These species were considerably larger, squat and toad-like leiopelmatids, which probably walked rather than hopped, and were found both on New Zealand’s North Island (L. markhami and L. waitomesis) and South Island (L. markhami and L. aurorensis). It is likely that all were wiped out by the (presumably accidental) introduction of the Pacific rat (Rattus exulans) by Polynesian settlers less than 1,000 years ago. The surviving species are restricted to the vicinity of North Island – Archey’s New Zealand frog and Hochstetter’s New Zealand frog occur only on the North Island, while the Maud Island frog and Hamilton’s frog are restricted to Maud Island in the Marlborough Sounds and Stephen’s Island in Cook Strait.

A primitive feature retained by Leiopelma frogs is their tail-wagging muscles (known scientifically as the caudalipuboischiotibialis muscles), although they no longer have a tail to wag. Other unique or unusual features of these frogs include the presence of elongate pieces of cartilage in the muscles of the abdomen (also called “inscriptional ribs”), round pupils, and an abnormally high number of vertebrae, or back bones – they have 9 presacral vertebrae with atypical concave ends, instead of the eight found in all other living frogs except their closest relatives, the tailed frogs from north-western U.S.A and Canada. Also, they cannot croak like most other species of frog (instead letting out a thin high-pitched squeak) because eardrums and vocal sacs have never developed in this group of frogs. Where aquatic in nature, they also swim differently to all other frogs (apart from the tailed frogs), in that they use alternate leg kicks which cause their heads to move from side to side during swimming in a rather energy inefficient way. It is thought that New Zealand frogs began their terrestrial (or ground dwelling) lifestyle before advanced swimming evolved in frogs. In keeping with their place as four of the world’s most primitive frogs, the way in which the skull remodels during metamorphosis within the egg, where the tadpole changes into a froglet, is said to be intermediate between a salamander and the tailed frogs (which are the next most primitive frogs on earth).

Hamilton’s frog is more closely related to Archey’s frog (EDGE rank 1) than to the other native New Zealand frogs.

Description

Despite being New Zealand’s largest native frog, Hamilton’s frog is a small species, with males reaching a total length of up to 43 mm and females being larger at 52 mm. They are mostly light brown in colour, although some green individuals have also been observed. A single dark stripe runs along each side of the head and through the eye. Defensive granular glands are present in the skin, which are concentrated into discrete patches arranged down the back and sides in about six long rows from head to rear, with the middle row being the most prominent. Glands are visible on the upper surface of the legs and feet, and to a lesser extent, the arms. There is no, or very little, webbing between the hind toes, and the fingers are not webbed.

Ecology

Hamilton’s frog is a ground-dwelling species that is nocturnal (night-active) and shelters in the damp crevices of its boulder bank habitat during the day. They can be difficult to locate because they are well camouflaged, nocturnal, do not croak and very rare.

During mating, the male clasps the female around the waist (a behaviour termed “inguinal amplexus”) and fertilises her eggs as they are laid. This species lays large unpigmented eggs in damp situations on the ground, which undergo direct development – New Zealand’s native frogs do not go through a tadpole stage, but instead develop totally within a gelatinous capsule in the egg, hatching out as froglets. They are therefore not dependent upon standing or flowing water for reproduction. The froglets take at least 3 to 4 years to reach maturity, and they retain a narrow tail fin throughout the early stages of their development outside of the egg. During reproduction the frogs, particularly males, occupy the oviposition (nesting) sites for weeks prior to the laying of eggs. The male Hamilton's frog guards the eggs until hatching and carries his young around on his back. Such care may keep the young moist, reduce predation and possibly reduce fungal or microbial infections. Despite not being reliant on water bodies for reproduction, Hamilton's frogs are very dependent on damp environments, and quickly dry out and die if placed in a dry place.

Hamilton’s frog, like all other native New Zealand frogs, does not exhibit a breeding call. This is because they are a primitive frog species that do not possess vocal chords or eardrums. However, they are known to squeak or chirp when annoyed, distressed, or during sexual activity. Because of the absence of well developed vocal sac, the sounds they produce depend upon resonance frequencies in head and body, rather than the vibration frequency of vocal chords.

The diet of Hamilton’s frog mainly comprises insects and other invertebrates. The main predator of this species on Stephen’s Island is the native tuatara – an unusual, ancient reptile which is rather similar to a small iguana in appearance, although it is highly distinct evolutionarily. As a defense mechanism, Hamilton’s frogs have been observed assuming a stiff-legged stance, rearing up and extending the legs. They can also remain motionless for long periods of time.

Habitat

Although its preferred habitat is likely to be moist forest, this species now survives in a damp, rocky pile that is covered mostly by grasses and shrubs near the summit of Stephens Island.

Distribution

This species is confined to a single rock stack (or boulder bank) on Stephens Island in the Marlborough Sounds area of New Zealand, where it is principally restricted to an area of 600 m2, which is one of the smallest known habitats for any frog species.
Translocation to an adjoining man-made rock stack on Stephens Island has been attempted but with limited success.

The total population of Hamilton’s frog is estimated to number less than 300 individuals. In May 2004, a small number were moved from Stephens Island to another predator-free rock stack island in the Marlborough Sounds.

Population Trend

The population of Hamilton’s frog is thought to be stable by the IUCN Red List of Threatened Species.

Status

Hamilton’s frog is listed as Endangered in the IUCN Red List of Threatened Species because its population size is estimated to number fewer than 250 mature individuals.

Threats

The major threats to Hamilton’s frog are introduced mammalian predators (e.g. the black rat, Rattus rattus) and disease, especially chytridiomycosis which has been recently identified in the closely related Archey’s frog in the North Island of New Zealand. Chytridiomycosis is caused by the chytrid fungus (Batrachochytrium dendrobatidis) that has been reported as a frog pathogen in many areas of the world. Chytrid fungus infection was first identified in Archey’s frog in September 2001 and has been the major causal factor in a number of amphibian extinctions worldwide. Also, a highly unusual native reptile called the tuatara (Sphenodon punctatus) may also prey on Hamilton’s frog. Furthermore, the small population and miniscule range of this species make it vulnerable to sudden decline and extinction brought on by sudden climatic change, or natural population fluctuations.

Conservation Underway

New Zealand has been protecting its indigenous amphibian species since 1921, when legislation was passed making it an offence to harm or remove frogs from their environment.

A tuatara-proof fence has been constructed around the habitat of Hamilton’s frog to reduce predation upon this very rare species. Regular population monitoring is also conducted so that any further declines may be swiftly detected. A Stephen’s Island habitat restoration programme is underway, and the prevention of accidental island colonisation by introduced mammalian predators is a priority action. Translocation of some frogs to a second island was conducted in May 2004 in an attempt to increase population numbers by extending the range of this species. The New Zealand Department of Conservation (DOC), acting through its Native Frog Recovery Group and Native Frog Recovery Plan, administers the conservation management of Hamilton’s frog and issues permits for appropriate species research.

A new man-made rock stack habitat for Hamilton’s frog was established between May and October 1991 in a nearby forest remnant 40 metres from the original site. The new habitat was created by evacuating pits and backfilling these with rocks, erecting a tuatara-proof fence around the site, and seeding the whole area with invertebrate prey. Twelve adult frog were originally transferred in 1992. These frogs appeared to adjust to the new area, but this project has been described as limited in its success, because the translocated population did not establish itself as well as was originally hoped and three frogs homed back to their original site. In 2004 a tuatara-proof fence was erected to protect these two sites and the intervening area for tuatara predation. In May 2004, forty frogs were translocated to another predator-free island called Nukuwaiata in the Marlborough Sounds. Their continued existence depends upon active conservation measures to protect this tiny population.

Conservation actions in the wild also consist of protecting habitat and removing mammalian predators (such as rats and mice) from frog habitat, or ensuring that they do not gain or increase access to existing frog populations. Further introduced frog species (there are already 3 species of non-native Australian treefrogs established in New Zealand) may out-compete native New Zealand frogs and/or introduce more virulent diseases, so it is also a priority of the DOC to ensure against the accidental introduction of other amphibian species.

Many conservation measures are underway for Hamilton’s frog. Projects such as those aiming to create more habitats for this species are extremely important given its tiny remnant range.The restoration of suitable habitat, such as the island’s native vegetation is a priority, as is the control of predators on the island (both native and introduced), and continued population monitoring of both numbers and disease. The only conservation measure that may be proposed for this species is that established actions continue, and that a captive breeding project be investigated.

Additionally, a huge amount of work is ongoing in New Zealand to investigate reasons for the decline of a close relative of Hamilton’s frog called Archey’s frog (the highest ranked EDGE amphibian species), including disease research, captive breeding, monitoring, population studies, and habitat conservation. The results of research carried out into the affects and treatment of chytrid will undoubtedly have positive repercussions for other species around the world, including Hamilton’s frog if chytrid is ever discovered in this species.

Bell, B.D., Daugherty, C.H. and Hay, J.M. 1998. Leiopelma pakeka, n.sp. (Anura: Leiopelmatidae), a cryptic species of frog from Maud Island, New Zealand, and a reassessment of the conservation status of L. hamiltoni from Stephens Island. Journal of the Royal Society of New Zealand 28: 39-54.